The present invention relates to a semiconductor device, particularly a heterojunction bipolar transistor.
Generally the bipolar transistor uses Si (silicon) as a material, but Si puts a physical limitation to the speedup of the bipolar transistor. In such circumstances, it is wished that a bipolar transistor using materials which are more effective for the speedup thereof will be developed. Such bipolar transistor is a heterojunction bipolar transistor (HBT) or the like having a base and a collector which are made of compound semiconductors, Ge (germanium) or others.
The HBT can have good emitter-base electron injection efficiency by using the heterojunction between the emitter and the base. In addition, the doping of the emitter layer and the base layer is not limited, and the design freedom is accordingly high. Consequently it is possible to make a device design suitable for speedup.
In this sense, the HBT is much expected to break the deadlock, it is said Si bipolar transistors have encountered in terms of speedup. Especially the HBT made of compound semiconductors is advantageous to ultraspeedup because of its superior electron transport characteristic in the base layer and the depletion layer of the collector layer and, in addition, further expands the design freedom including the band structure. This HBT is being vigorously studied.
There is a tendency that compound semiconductors are more effective for the speedup of the bipolar transistor as they have lower bandgap energy Eg. The bandgap energy Eg of InGaAs is about half that of GaAs. It is very effective for the speedup of the bipolar transistor to make the base and the collector of compound semiconductors having such narrow bandgaps. But on the other hand, the voltage resistance between the base and the collector is adversely lowered, which resultantly causes troubles to the circuit operation, etc. In view of this, the high base-collector breakdown-voltage is needed.
In the bipolar transistor, the breakdown voltage between the base and the collector is determined by the breakdown of a p-n junction between the base and the collector. For the occurrence of this breakdown, both the tunnel effect and the avalanche effect are important mechanisms, and usually the avalanche effect determines the breakdown voltage.
That is, as shown in the base-collector band structure of FIG. 1, electrons injected from a base layer 102 into a collector depletion region 104 have high kinetic energy Ek, exposed to an intense electoric field or others, and collide with lattice atoms. As the electrons flow further to the collector layer 106, the kinetic energy Ek of these electrons increases. When the kinetic energy Ek has an intensity which excites the electrons from the valence band to the conduction band, or excites holes from the conduction band to the valence band, i.e., becomes high enough to jump a bandgap Eg, bonds of the lattice atoms are cut to generate electron-hole pairs. These generated electrons and holes have energy, exposed to the electric field and generate different new electron-hole pairs. This process repeatedly takes place to cause an avalanche effect, and the p-n junction between the base and the collector is broken.
To suppress the occurrence of the breakdown caused by the avalanche effect, the bandgap Eg in the depletion region of the p-n junction is made large.
That is, as shown in the base-collector band structure of FIG. 2, a collector layer 112 is made of a semiconductor material having a wider bandgap Eg than that of a base layer 110. For the prevention of spikes of the band Ec between the base layer 110 and the collector layer 112, there is provided a graded layer 114 having a gradually changing bandgap. The other side of the collector layer 112, whose bandgap is wider, is jointed to a sub-collector layer 118 with a narrower bandgap doped with a high concentration of an impurity through a graded layer 116 with a gradually changing bandgap doped with a high concentration of an impurity.
In the case the base layer 110 is made of, e.g., Ge, the collector layer 112 is made of SiGe or others. In the case the base layer 110 is made of, e.g., InGaAs, the collector layer 112 is made of InAlAs or others.
But for the purpose of realizing high breakdown-voltage of the collector, it is contrary to the intention of realizing the speedup of the bipolar transistor by making the base and the collector of semiconductor materials having narrow bandgaps to make, as conventionally done, the collector layer of semiconductor materials having wide bandgaps. In other words, the base layer alone is made of a semiconductor material with a narrow bandgap, and accordingly the speedup of the operation of the bipolar transistor cannot be realized. Thus, the realization of high breakdown-voltage of the collectors by the conventional method involves a problem that the speedup of the bipolar transistor has to be sacrificed.
In the case the base layer is made of InGaAs, and the collector layer is made of InAlAs, since an energy level difference .DELTA.E.sub..GAMMA.-L between .GAMMA.-valley and L-valley in InAlAs is relatively as small as 0.23 eV, electrons injected from the base layer into the collector layer to easily transferred from .GAMMA.-valley to L-valley, so, velocity overshoot is not so effective with the result of decrease of the velocity of the electrons. Accordingly the increase of the velocity of the electrons due to the velocity overshoot effect cannot be realized, and it is a problem that the speedup of the bipolar transistor cannot be realized.
On the other hand, the HBT using AlGaAs/GaAs heterojunction out of various compound semiconductors has been much studied also because of its easily controllable crystal growth. As a result this HBT has the fastest switching speed record of the presently used semiconductor devices.
What is more important in terms of the design of the device structure for further super-speedup is to reduce a transit time in the base layer and the collector depletion layer as well as reduction of parasitic capacitance and parasitic resistance. The reduction of a transit time in the base layer is realized by, e.g., thinning the base layer and grading the base layer, and the reduction of a transit time in the collector depletion layer is realized by optimizing the electric field in the depletion layer by using a p-type collector, an i-type collector or the structure of a BCT (Ballistic Collection Transistor).
FIG. 3 shows band structures and layer structures of a usual HBT. FIG. 4 shows band structures and layer structures of a BCT using i/p.sup.+ /n.sup.+ structure as the high-speed collector structures.
These HBT and BCT are common in n-type emitter layers 122, 132, and p.sup.+ -type base layers 124, 134, but an n-type collector layer 126 of the HBT corresponds to a collector layer of the BCT comprising an i-type layer 136, p.sup.+ -type planar doped layer 138 and an n.sup.+ -type layer 140.
In the i/p.sup.+ /n.sup.+ multi-layer collector structure of the BCT, the electric field in the i-type layer 136 is optimized by adjusting the concentration of the p.sup.+ -type planar doped layer 138 so that electrons velocity-overshoot and transit "quasi-ballistically" in the entire region of the i-type layer 136 having a lower concentration of an impurity.
That is, in the HBT of FIG. 3 having the usual n-type collector structure, electrons injected from the p+-type base layer 124 into the n-type collector layer 126 are immediately injected into the L-valley, while in the BCT of FIG. 4 having the i/p.sup.+ /n.sup.+ multi-layer collector structure, electrons velocity-overshoot in almost the entire region of the collector layer within a certain collector voltage Vce, and now the electrons can quasi-ballistically transit in the .GAMMA.-valley where the transit speed is higher than that in the L-valley. Accordingly by using the i/p.sup.+ /n.sup.+ multi-layer collector structure, the electron transit time mainly causing the retardation of the device, especially the electron transit time in the collector depletion layer can be decreased.
For example, a report says that the i/p.sup.+ /n.sup.+ multi-layer collector structure was adapted to the HBT using AlGaAs/GaAs heterojunction, whereby a maximum cut-off frequency of 105 GHz was realized (T. Ishibashi et al., "ULTRA-HIGH SPEED AlGaAs/GaAs HETEROJUNCTION BIPOLAR TRANSISTOR", 1988 International Electron Devices Meeting TECHNICAL DIGEST, pp 826-829). This report have evidenced the utility of this structure.
But, in the case the base layer is made of GaAs, a turn-on voltage becomes high, and accordingly the source voltage becomes high, power consumption becoming large. Thus it is difficult to integrate many HBTs.
On the other hand, a HBT using the InAlAs/InGaAs or InP/InGaAs systems having base layers made of InGaAs with a narrow bandgap, have, in comparison with the HBT having base layers made of GaAs, higher electron mobility and higher speed, and also lower turn-on voltage and accordingly low power consumption.
Here, for lower power consumption, when the above-mentioned BCT structure is applied to the HBT having a base layer with a narrow bandgap, such as the InAlAs/InGaAs HBT or the InP/InGaAs HBT, the collector structure of the HBT becomes the i-type InGaAs/p.sup.+ -type InGaAs/n.sup.+ -type InGaAs system. InGaAs has such a high electron mobility and a large energy difference between the .GAMMA.-valley and the L-valley that the velocity-overshoot works more effectively.
Thus, it is considered very effective for the speedup to apply the BCT structure having an i/p.sup.+ /n.sup.+ multi-layer collector to the HBT having the InGaAs base with a narrow bandgap.
As above described, though it is very effective to the speedup to apply the BCT structure having an i/p.sup.+ /n.sup.+ multi-layer collector to the HBT having the InGaAs base with a narrow bandgap, it causes the following problem.
That is, as seen in FIG. 4, the electric field in the depletion layer between p.sup.+ -type planar doped layer 138 and a n.sup.+ -collector layer 140 is more intense than that in the collector depletion layer of the usual collector structure. Accordingly the breakdown voltage of this collector structure is determined by a p.sup.+ -n.sup.+ junction between the p.sup.+ -type planar doped layer 138 and the n.sup.+ -type collector layer 140 and is lower than that of the usual collector structure.
Furthermore, in the case InGaAs with a narrow bandgap is used in the base layer, it is usual to make the collector layer of InGaAs. Narrow bandgap semiconductors such as InGaAs, whose ionization ratio is high, tend to cause the avalanche effect and also the tunnel effect. The collector breakdown voltage is further lowered in the case the collector layer is made of semiconductor with a narrow bandgap.
Thus, the problem is that the decrease of the breakdown voltage characteristic of the collector greatly limits the circuit structure, making the circuitry operation difficult.
As a solution of this problem, it can be proposed to use a double heterostructure for widening the bandgap of the collector layer in the case the base layer is made of a narrow bandgap semiconductor, but the transit time of electrons in the collector depletion layer is accordingly impaired, sacrificing the speedup.